The goal of this project is to better understand the process of naturally occurring neuronal death that plays a widespread role in normal mammalian development. The trophic interaction between afferent and target neuronal populations is relevant not only to human developmental disorders, but may also be an important element in degenerative diseases of aging. The model system selected for the proposed research is the granule cell-Purkinje cell circuit in the mouse cerebellum. There are two specific aims. The first is to determine when the period of target dependence actually commences, and how much cell death normally occurs while granule cell proliferation is still underway. Both the magnitude and timing of granule cell death will be accurately determined in normal animals, and in staggerer mutants that lack competent target from the outset. To do this, cohorts of granule cells having selected birthdates will be labeled in groups of mice by 3H-thymidine uptake and the fraction of labeled cells remaining will be assessed by autoradiography at subsequent times. A related issue concerns whether granule cells must reach a certain developmental threshold as measured by axon outgrowth before they become vulnerable to cell death. This question will be addressed by comparing the distribution of parallel fiber lengths in normal mice with that in target deficient mutants (lurcher and staggerer), using focal implants of the lipophilic fluorescent dye DiI in fixed cerebella. The second aim is to explore a hypothesis involving developmental deafferentation in lurcher<->wild type mouse chimeras. The hypothesis holds that an altered trophic environment is induced by the early death of mutant Purkinje cells and the subsequent deafferentation of their wild type counterparts, and that the altered environment is able to rescue a fraction of granule cells from cell death. This is interesting for two reasons: it suggests a feedback relationship during development, nd it suggests a role for these chimeras in identifying the trophic molecule. Lurcher chimeras will be constructed that incorporate an existing transgenic mouse line containing a Purkinje cell specific histochemical marker. These chimeras will extend our understanding of the aberrant Purkinje cell morphologies that characterize these mice and are key to the hypothesis. Also, a developmental electron microscopic study will determine whether the onset of these morphologies is consistent with the deafferentation hypothesis.